The present invention relates to a coordinate reading apparatus.
The coordinate reading apparatus, to which the present invention refers, comprises, at least, a tablet, a position detector which detects a specified position to be read and a processing apparatus which produces coordinate data with respect to the specified position. Such coordinate reading apparatus is useful as a graphic information processing system which is very important in the technical field of, for example, automatic design or automatic drawing.
According to the coordinate reading apparatus of the prior art, three kinds of methods have been proposed and put into practical use, for producing a signal which indicates relative location between the tablet and the position detector. First of the methods is a method based on an electrostatic coupling between the tablet and the position detector, second thereof is a method based on an electromagnetic induction therebetween, third thereof is a method based on an ohmic resistance obtained on the tablet. To be more specific, according to the first method which is based on the electrostatic coupling, alternating signals, corresponding to the coordinate values, are applied to the tablet, and each of the alternating signals is picked up via a dielectric layer, based on the electrostatic coupling, by means of a position detector. The "Sylvania Data Tablet" (explained hereinafter) is one example according to the first method. According to the second method, based on electromagnetic induction, pulse signals are applied from an object, for example a pen, and the pulse signals are picked up by a position detector located on the tablet. According to the third method, based on ohmic resistance, resistance wires are distributed on the tablet, and the resistance value of the resistance wire, corresponding to the coordinate values, is picked up by a position detector located on the tablet. The coordinate reading apparatus of the present invention is basically constructed according to the above mentioned electromagnetic induction method, however, it does not have the same construction as the above mentioned construction, based on the prior art electromagnetic induction method. That is, the coordinate reading apparatus of the present invention utilizes, as will be explained hereinafter, a rotating magnetic field.
In such a coordinate reading apparatus, there are two important conditions to be satisfied therein. The first condition is that the position detector must produce an analogue electric signal which is correctly proportional to each coordinate value with a high degree of accuracy. Second is that the analogue electric signal must be constant at any angle between a ball-pen type position detector and the tablet, when the detector is located at a desired coordinate position on the tablet. It should be noted that it is very difficult for an operator to stand the ball-pen type position detector always at right angles with respect to the tablet from an operational view point. Regarding a technique for satisfying the above-mentioned former condition, usually, the resolution of the coordinate of the tablet is increased so as to detect the desired coordinate value with a high degree of accuracy. However, the technique for increasing the resolution produces, on the other hand, a first defect in that since the increase of the resolution requires an increase in amount of hardware, this technique is not preferable from an economical view point. Regarding a technique for satisfying the above-mentioned latter condition, usually, a particular means for detecting a tilt angle of the position detector is employed, and the analogue electric signal, produced from the position detector, is compensated by a detected tilt angle signal from the particular means, in which the particular means and the position detector work as one body. However, the technique for employing the particular means produces, on the other hand, a second defect in that firstly the employment of such particular means is not preferable from an economical view point and secondly, it is not easy to achieve the compensation of the analogue electric signal, by the tilt angle signal, with a high degree of accuracy.
In order to overcome the above-mentioned first defect, according to the aforesaid "Sylvania Data Tablet," a concept called phase detection is taken into consideration. That is, the coordinate data is obtained by detecting a variation of the phase of the analogue electric signal produced from the position detector. According to this method, the coordinate reading can be achieved, with a high degree of resolution and without increasing the amount of hardware, by detecting a continuous variation of the phase of the analogue electric signal. However, according to the Sylvania Data Tablet, there exists a problem in that the analogue electric signal varies due to the presence of an external dielectric object including a human body, because the Sylvania Data Tablet is constructed by utilizing the electrostatic coupling method. Further, the Sylvania Data Tablet employs no means for overcoming the above-mentioned second defect. Furthermore, generally, a suitable means for overcoming such second defect, has not been previously proposed.